Anchorage-dependent cell cultivation requires provision of an attachment surface onto which the cultivated cells may anchor and grow. Cell production depends, among other factors, upon the amount of surface area available for cell attachment. It has heretofore been proposed to increase available surface area by replacing standard roller bottle and petrie dish glass attachment surfaces with a stationary bed of solid glass beads (density .congruent.2.3 g/cc) around and through which the nutrient medium is continuously circulated. It has also been proposed to increase attachment surface-to-volume ratio, and thus increase production efficiency, by employing so-called microcarriers which remain in suspension under continuous agitation in the culture medium.
Most typically, the proposed microcarriers are in the form of porous plastic (dextran) beads. Plastic microcarriers of this type require alteration of electrically charged surface moieties to promote cell attachment, which alteration is difficult to control quantitatively in production, and is toxic to some fastidious types of cell cultures if not properly controlled. The porous plastic beads absorb cell nutrients and metabolic wastes. Moreover, cell pseudopods attach and cling to pores and surface irregularities, making harvesting of the cells and cleansing and reuse of the plastic microcarriers most difficult. The art relative to provision of attachment surfaces for anchorage-dependent cell cultivation is surveyed in Levine et al, "Optimizing Parameters for Growth of Anchorage-Dependent Mammalian Cells in Microcarrier Cultures", Cell Culture and its Application, Acton ed., Academic Press (1977), pp. 191-216, and in 3rd General Meeting of ESACT, Oxford 1979, Develop. Biol. Standard, 46, pp. 109-294 (S. Karger, Basel 1980).
In the copending Downs et al., U.S. application Ser. No. 332,377 filed Dec. 21, 1981 and assigned to the assignee hereof, the foregoing and other difficulties in the microcarrier arts are addressed by forming hollow precursor microspheres of silicate glass composition, and then tailoring the density of such precursor microspheres in a postforming etching operation to match closely the density of the desired aqueous growth medium, usually in the range of 1.01 to 1.09 g/cc. Microcarriers so formed have proven successful in overcoming the surface-charge, buoyancy, harvesting and reuse problems of the earlier art. However, the number of separate manufacturing operations involved has made cost reduction desirable.
U.S. Pat. No. 4,448,884 to the inventor herein and assigned to the assignee hereof discloses a further step forward in the art wherein microcarriers for anchorage-dependent cell cultivation are prepared to contain a spherical substrate of polymeric material having a bulk density of about 1 g/cc so as to be substantially buoyant in an aqueous culture medium, and a thin (less than 1 .mu.m) coating layer of silicate glass. The silicate glass coating layer is preferably applied to a spherical precursor of polymeric material in an rf sputtering operation. An intermediate coating layer of magnetic material may be deposited prior to the silicate glass layer, so that the microcarriers may be readily removed from culture media by suitable subjection to a magnetic field.